1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting and receiving packets in a mobile communication system, and in particular, to a method and apparatus including several upper layer packets in one lower layer packet before transmission in order to reduce header overhead.
2. Description of the Related Art
In the next generation mobile communication systems, a base station schedules data transmission/reception in real time. For example, a terminal or User Equipment (UE) reports the data generation and channel condition to a base station (Step 1). The base station identifies a UE that will transmit data in the next transmission period, and determines transmission resources to be allocated to the UE, based on the data-generated state and channel state of several UEs (Step 2). Thereafter, the base station allocates the transmission resources to the UE (Step 3). In this case, the UE should undergo a preliminary operation of the above three steps in order to transmit uplink data. However, this is inefficient for the service in which a packet of several tens of bytes is periodically frequently generated, such as Voice over Internet Protocol (VoIP).
FIG. 1 illustrates an example of bundling 2 VoIP packets in one Radio Link Control Packet Data Unit (RLC PDU). For reference, RLC is a protocol layer in charge of framing for reassembling an upper layer packet in a size suitable for transmission over a radio channel. PDU is a packet output from an arbitrary protocol layer device, and Service Data Unit (SDU) is a packet being input to an arbitrary protocol layer device.
When two RLC SDUs 105 and 110 are contained in on one RLC PDU 160 and each RLC SDU is one VoIP packet, a sequence number 120, length indicators 130 and 140, and Extension (E) bits 125, 135, and 145 are contained in a header 115 of the RLC PDU.
The sequence number 120 increases by one for each RLC PDU so that a reception device of RLC PDU knows the sequence relation between received RLC PDUs. Normally, the sequence number 120 has a size of about 1 byte. The length indicators 130 and 140 indicate to which byte in an RLC PDU payload the last byte of the RLC SDU contained in the RLC PDU corresponds. The number of length indicators in one RLC PDU is determined depending on the number of RLC SDUs contained in the RLC PDU. The E bits 125, 135, and 145 are flags indicating whether the next field is a pair of a length indicator and an E bit, or data.
For example, when an RLC SDU 105 with a 35-byte size and an RLC SDU 110 with a 37-byte size are contained in the RLC PDU 160, the first length indicator 130 of the RLC PDU indicates 35 and the second length indicator 140 indicates 72.
The length indicator can also indicate an interval between the last bytes of RLC SDUs, rather than the information indicating a position of the last byte of the RLC SDU in the RLC PDU. In this case, the length indicator can also indicate a size of the RLC SDU. When the length indicator is used to indicate a size of the RLC SDU in this way, the exemplary second length indicator 140 indicates 37.
As described above, as many length indicators as the number of simultaneously containable VoIP packets are inserted into one RLC PDU. Given that the size of the length indicator is 1 byte or 2 bytes, overhead by the length indicator is ignorable. For example, in VoIP where a codec having a low data rate of 4.75 kbps is used, because a VoIP packet of about 17 bytes is mainly generated, overhead by the length indicator of 1-2 bytes reaches 6% or 12%.
Therefore, there is a need for a scheme that reduces overhead of length indicators when small-sized packets, such as VoIP packets, are simultaneously contained in one PDU.